Type mask for combining off axis illumination and attenuating phase shifting mask patterns

ABSTRACT

A mask and method of forming a mask for forming electrode patterns having both closely spaced lines and lines with greater separation between them. The mask uses a pattern formed using attenuating phase shifting material for the region of the mask with lines with greater separation and a binary pattern formed using opaque material in the region of the mask with closely spaced lines. The mask design data is used to determine the mask regions using attenuating phase shifting material and the regions of the mask using a binary pattern. The mask is illuminated using off axis illumination, preferably quadrapole off axis illumination. The mask is formed using electron beam exposure of a resist using more than one exposure dose so that only one layer of resist is required to form the two regions of the mask one using attenuating phase shifting material and one using a binary pattern.

This is a division of patent application Ser. No. 09/097,144, filingdate Jun. 12, 1998 now U.S. Pat. No. 6,150,058, A New Type Mask ForCombining Off Axis Illumination And Attenuating Phase Shifting MaskPatterns, assigned to the same assignee as the present invention.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention relates to the use of masks using attenuating phaseshifting material for regions of the pattern with relatively largespaces between lines and binary mask patterns using opaque material forregions of the pattern with relatively small spaces between lines on thesame mask substrate. The mask is illuminated using off axis illuminationwhen using the mask to form a pattern on an integrated circuit wafer.The mask is formed using a single resist layer exposed using multipleelectron beam exposure doses.

(2) Description of the Related Art

U.S. Pat. No. 5,481,332 to Shiraishi describes a projection exposuremethod and apparatus for improved image transfer from photomasks havingperiodic and non-periodic patterns. The projection optical system usesoff axis illumination to illuminate the photomask.

U.S. Pat. No. 5,503,951 to Flanders et al. describes a method of forminga photomask using attenuating phase shifting material.

U.S. Pat. No. 5,480,747 to Vasudev describes a method of forming aphotomask using attenuating phase shifting material and using embeddedabsorbers.

A patent application ( ) entitled “A NEW DOUBLE LAYER METHOD FORFABRICATING A RIM TYPE ATTENUATING PHASE SHIFTING MASK”, Ser. No.08/956,971, Filed on Oct. 3, 1997, and assigned to the same assigneedescribes a method of forming multiple patterns in a single layer ofresist using two different electron beam exposure doses.

A patent application ( ) entitled “A MASK AND SIMPLIFIED METHOD OFFORMING A MASK INTEGRATING ATTENUATING PHASE SHIFTING MASK PATTERNS ANDBINARY MASK PATTERNS ON THE SAME MASK SUBSTRATE”, Ser. No. 09/020,502,Filed on Feb. 9, 1998, and assigned to the same assignee describes abinary mask pattern and a rim type attenuating phase shifting maskpattern formed on the same transparent mask substrate.

A paper entitled “Optimization of the optical phase shift in attenuatedPSM and application to quarter micron deep-UV lithography for logics” byK. Ronse et al., SPIE Vol. 2197, pages 86-98 discusses opticaltechniques to achieve optimization of attenuated phase shifting masks.

A paper entitled “Quarter Micron Lithography System with ObliqueIllumination and Pupil Filter” by S. Orii et al., SPIE Vol. 2197, pages854-868 discusses the usefullness of pupil filtering.

A paper entitled “Dependency of side-lobe effect of half-tone phaseshift mask on substrate material and topology, and its solutions” bySung-Chul Lim et al., SPIE Vol. 2512, pages 372-383 discusses loss ofresist due to side lobe effect.

SUMMARY OF THE INVENTION

In the manufacture of semiconductor integrated circuit wafersphotolithographic processing methods having good resolution and a largedepth of focus are required to form fine lines. The need for very closespacing between the fine lines places added requirements on thephotolithographic processing. In regions of the pattern where thespacing between the fine lines is relatively large the use of masksusing attenuating phase shifting material produces good results. Inthose regions of the pattern where the spacing between the fine lines issmall, however, attenuating phase shifting material will not give goodresults due to side lobe effect.

In regions of the pattern where the spacing between the fine lines issmall improved results can be obtained using binary masks and off axisillumination, OAI, of the masks. In the case where the parallel linesare in predominantly one direction dipole illumination using off axisillumination works well. In those cases where the parallel lines are inmore than one direction quadrapole illumination using off axisillumination will be required.

It is a principle objective of this invention to provide a method offorming a mask having both attenuating phase shifting patterns andbinary patterns on the same mask.

It is another principle objective of this invention to provide a methodof exposing a layer of photoresist on a substrate using quadrapole offaxis illumination of a mask having both attenuating phase shiftingpatterns and binary patterns on the same mask.

It is another principle objective of this invention to provide a maskhaving both attenuating phase shifting patterns and binary patterns onthe same mask.

These objectives are achieved by forming a layer of attenuating phaseshifting material on a transparent mask substrate. A layer of opaquematerial is formed on the layer of attenuating phase shifting material.The patterns are then formed in both the layer of opaque material andthe layer of attenuating phase shifting material. In that part of thepattern comprising lines and spaces wherein the ratio of the width ofthe spaces between lines to the line width is large the opaque materialis removed from the pattern and the pattern is formed using attenuatingphase shifting material. In that part of the pattern comprising linesand spaces wherein the ratio of the width of the spaces between lines tothe line width is small the opaque material is left in place and forms abinary pattern. The design data used to form the mask is analyzed todetermine the regions of the mask in which the opaque material isremoved and which regions of the mask the opaque material is left inplace.

The mask is then used in a projection system to form the image of themask on an integrated circuit wafer having a layer of photoresist formedthereon. The mask is illuminated using off axis illumination, preferablyquadrapole off axis illumination, and the light passing through the maskis focussed on the layer of photoresist formed on the integrated circuitwafer.

In one embodiment the mask is formed using a single layer of resist. Alayer of attenuating phase shifting material is formed on a transparentmask substrate and a layer of opaque material is formed on the layer ofattenuating phase shifting material. A layer of resist is formed on thelayer of opaque material. The region of the mask where the ratio of thespacing between the lines to the line width is large is exposed with afirst exposure dose and the region of the mask where the ratio of thespacing between the lines is small is exposed with a second exposuredose. The resist is then developed leaving a thinner resist pattern inthe region of the mask where the ratio of the spacing between the linesto the line width is large. That part of the opaque material andattenuating phase shifting material not covered by resist is then etchedaway. The resist is then partially etched away thereby removing resistfrom the region of the mask where the ratio of the spacing between thelines to the line width is large. The opaque material is then etchedaway from the region of the mask where the ratio of the spacing betweenthe lines to the line width is large leaving a pattern formed ofattenuating phase shifting material only. The remaining resist is thenremoved and the mask is completed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a top view of an electrode pattern an a segment of anintegrated circuit wafer.

FIG. 2 shows a cross section view of the integrated circuit wafersegment of FIG. 1 along line 2-2′ of FIG. 1.

FIG. 3 shows a top view of a mask used in forming the electrode patternof FIG. 1 on an integrated circuit wafer.

FIG. 4 shows a cross section view of the mask of FIG. 3 along line 4-4′of FIG. 3.

FIG. 5 shows a schematic diagram of a optical projection system used inphotolithographic processing of integrated circuit wafers.

FIG. 6A shows an aperture used for quadrapole off axis illumination of amask.

FIG. 6B shows an aperture used for dipole off axis illumination of amask.

FIG. 7 shows a cross section view of a mask substrate with a layer ofattenuating phase shifting material, a layer of opaque material, and alayer of resist formed thereon.

FIG. 8 shows a cross section view of the mask of FIG. 7 after the layerof resist has been exposed and developed.

FIG. 9 shows a cross section view of the mask of FIG. 8 after the layerof opaque material has been etched.

FIG. 10 shows a cross section view of the mask of FIG. 9 after the layerof attenuating phase shifting material has been etched.

FIG. 11 shows a cross section view of the mask of FIG. 10 after part ofthe resist mask has been etched away.

FIG. 12 shows a cross section view of the mask of FIG. 11 after theopaque material has been etched away from that part of the pattern inthe region where the ratio of the space between the lines to the linewidth is large.

FIG. 13 shows a cross section view of the completed mask after theremaining resist has been removed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Refer now to FIGS. 1-6B for a description of the preferred embodimentsof the mask of this invention and the method of this invention ofexposing a layer of photoresist on an integrated circuit wafer usingmask having a pattern of both closely spaced and widely spaced lines onthe same mask. FIG. 1 shows a top view of a segment of an integratedcircuit wafer 10 showing an electrode pattern comprising a number ofparallel lines, 12 and 14. The electrodes each have a line width 16. Ina first part of the pattern the space between the electrodes 18 isrelatively large and in a second part of the pattern the space betweenthe electrodes 20 is relatively small. FIG. 2 shows a cross section viewof the electrode pattern along line 2-2′ of FIG. 1.

FIG. 3 shows a top view of a segment of a mask of this invention used toform the electrode pattern of FIGS. 1 and 2 on the integrated circuitwafer. FIG. 4 shows a cross section view of this mask segment takenalong line 4-4′ of FIG. 3. The mask has a first pattern region 37 wherethe ratio of the spacing 33 between lines 32 to the line width 31 isrelatively large, greater than about 2.0, see FIG. 3. In the firstpattern region the lines 32 are formed of attenuating phase shiftingmaterial such as MoSiO having a thickness of between about 900 and 1600Angstroms. The TSMC-97-276 thickness of the MoSiO is dependent on thetechnology, transmittance of the attenuating phase shifting material,and the vendor of the mask blanks. The mask also has a second patternregion 39 where the ratio of the spacing 35 between lines 34 to the linewidth 31 is relatively small, less than about 2.0, see FIG. 3. In thesecond pattern region the lines 34 are opaque. In this example the linesin the second pattern region are formed of an opaque material 34, suchas chrome having a thickness of between about 600 and 1200 Angstroms,formed over lines formed of attenuating phase shifting material, seeFIG. 4.

This mask provides a first pattern and a second pattern on the same masksubstrate. The first pattern is formed of attenuating phase shiftingmaterial in the first pattern region where the ratio of the spacing 33between lines 32 to the line width 31 is relatively large and side lobeeffect is not a problem. The second pattern is formed of opaquematerial, thereby forming a binary mask segment, in the second patternregion where the ratio of the spacing 35 between lines 34 to the linewidth 31 is relatively small and side lobe effect would be a problem ifattenuating phase shifting material were used. The mask design data isused to determine which sections of the mask are formed usingattenuating phase shifting material and which sections of the mask willhave a binary pattern formed with opaque material. In order to obtaingood resolution and depth of focus in the second pattern region the maskis illuminated using off axis illumination.

FIG. 5 shows a schematic diagram of an optical projection system used toexpose a layer of photoresist in an integrated circuit wafer with themask pattern. FIG. 5 shows a light source 20, an exit aperture 22 fromthe light source, a means 23 for holding the mask 31, a lens 24 to focusthe light passing through the mask 31, and a wafer holder 25 holding anintegrated circuit wafer 10 with a layer of photoresist 11 formedthereon. The projection system has an optical axis 26 which passesthrough the center of the lens 24. As shown in FIG. 5, the mask 31 isbetween the exit aperture 22 and the lens 24 and the lens 24 is betweenthe mask 31 and the layer of photoresist 11 on the integrated circuitwafer 10. In order to illuminate the mask 31 the light must pass throughthe exit aperture 22. The mask is the mask described above having afirst pattern of attenuating phase shifting material and a secondpattern of opaque material forming a binary mask.

Two possible exit apertures 22 are shown in FIGS. 6A and 6B. Thepreferred exit aperture is shown in FIGS. 6A and shows an opaque panelwith four openings 28 located away from the optical axis 26. Theopenings 28 are all an equal distance away from the optical axis 26 andlocated at the four corners of a square. This exit aperture providesquadrapole off axis illumination of the mask 31. Another possible exitaperture is shown in FIG. 6B and has an opaque panel with two openings28. The two openings 28 are an equal distance away from the optical axis26 and the two openings and the optical axis are co-linear. This exitaperture provides dipole off axis illumination of the mask 31. Theseexit apertures, FIGS. 6A and 6B, provide illumination of the photomaskfrom oblique directions, at a small angle from the optical axis, whichprovides improved resolution and depth of field for patterns which havea number of closely spaced parallel lines.

Refer now to FIGS. 7-13 for the preferred embodiment of a method offorming the mask of this invention. FIG. 7 shows a cross section view ofa transparent mask substrate 30. A layer of attenuating phase shiftingmaterial 32, such as MoSiO having a thickness of between about 300 and1600 Angstroms. A layer of opaque material 34, such as chrome having athickness of between about 600 and 1200 Angstroms, is deposited on thelayer of attenuating phase shifting material. A layer of resist 36 isformed on the layer of opaque material 34. The layer of resist iscomposed of a first thickness 44 at the top of the layer of resist 36and a second thickness at the bottom of the layer of resist 36. A firstpattern will be formed in the layer of resist 36 in the regions 40 wherethe ratio of the spacing 33 between lines 32 to the line width 31 isrelatively large, greater than about 2.0, see FIG. 3. A second patternwill be formed in the layer of resist 36 in the regions 42 where theratio of the spacing 33 between lines 32 to the line width 31 isrelatively small, less than about 2.0, see FIG. 3. The layer of resist36 is exposed using an electron beam 38 using with different exposuredoses.

As shown in FIG. 8, the layer of resist is then developed. The exposureof the layer of resist is such that after developing the layer of resist36, the first pattern will be formed in only the second thickness 46 ofthe layer of resist and the second pattern will be formed in both thefirst thickness 44 and second thickness 46 of the layer of resist. Asshown in FIG. 9, that part of the layer of opaque material not coveredby resist is etched away using either wet or dry isotropic etching,preferably wet isotropic etching, following methods well established inmask fabrication technology. Next, as shown in FIG. 10, that part of thelayer of attenuating phase shifting material not covered by resist isetched away using dry anisotropic etching following methods wellestablished in mask fabrication technology.

Next, as shown in FIG. 11, part of the resist is etched away using dryanisotropic etching. The dry anisotropic etching removes an amount ofresist equivalent to the second thickness so that all the resist isremoved from the first pattern of the layer of resist the regions 40where the ratio of the spacing between lines to the line width 31 isrelatively large. This leaves the second pattern formed in a firstthickness of the layer of resist 36 in the regions 42 where the ratio ofthe spacing between lines to the line width is relatively small. Next,as shown in FIG. 12 any opaque material not covered by resist is etchedaway using either wet or dry isotropic etching, preferably wet isotropicetching, following methods well established in mask fabricationtechnology. As shown in FIG. 13 the remaining resist is then removed andthe mask is completed. The mask design data is used to determine whichsections of the mask are formed using attenuating phase shiftingmaterial and which sections of the mask will have a binary patternformed with opaque material.

While the invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of the invention.

What is claimed is:
 1. A photomask, comprising: a transparent masksubstrate having a first region and a second region; a first patternformed of attenuating phase shifting material on said first region ofsaid transparent mask substrate, wherein said first pattern comprisesparallel lines having a first distance between adjacent lines and eachsaid line has a line width of a second distance; a second pattern formedof said attenuating phase shifting material on said second region ofsaid transparent mask substrate, wherein said second pattern comprisesparallel lines having a third distance between adjacent lines and eachsaid line has a line width of a fourth distance and wherein the ratio ofsaid fourth distance to said third distance is greater than the ratio ofsaid second distance to said first distance; and opaque material formedon said attenuating phase shifting material forming said second pattern.2. The photomask of claim 1 wherein the ration of said second distanceto said first distance is less than about 2.0.
 3. The photomask of claim1 wherein the ratio of said fourth distance to said third distance isgreater than about 2.0.
 4. The photomask of claim 1 wherein said opaquematerial is chrome having a thickness of between about 600 and 1200Angstroms.
 5. The photomask of claim 1 wherein said layer of attenuatingphase shifting material is MoSiO having a thickness of between about 900and 1600 Angstroms.
 6. The photomask of claim 1 wherein said transparentmask substrate is quartz having a thickness of about 0.25 inches.